1. Field of the Invention
This invention relates to a fuel supply system for an internal combustion engine (hereinafter, referred to as engine).
2. Description of Related Art
There have been fuel supply systems which keep fuel supplied by a fuel pump to fuel injectors at a predetermined pressure by means of a pressure regulator disposed in a fuel supply passage leading from the fuel pump to the injectors.
An example of a conventional pressure regulator of a fuel supply system of this kind, such as disclosed in JP-A-62162767, is shown in FIG. 8. Referring to FIG. 8, a pressure regulator 110 comprises a diaphragm 113 clamped at a boundary between a first case 111 and a second case 112. The central portion of the diaphragm 113 is gripped between a valve part and a spring receiver 114, and the diaphragm 113, the valve part 120 and the spring receiver 114 reciprocate integrally. The valve part 120 is urged in a valve-closing direction by a compression coil spring 115.
A spring chamber 111a inside the first case 111 is connected to an air intake pipe not shown in the figure by a connecting pipe 116, and the spring chamber 111a is thereby set at an air intake pipe negative or vacuum pressure. An inlet pipe 131 through which fuel is introduced into a fuel chamber 112a from a fuel tank not shown in the figure and a return pipe 132 through which excess fuel is returned to the fuel tank are connected to the second case 112.
The valve part 120 displaces to a position such that the air intake pipe negative pressure of the spring chamber 111a, the fuel pressure of the fuel chamber 112a and the urging force in the valve-closing direction of the compression coil spring 115 are in equilibrium. When fuel is introduced from the fuel pump, the valve part 120 lifts according to the fuel delivery flow rate from the fuel pump and a valve-opening area formed between a valve body 121 and a seat member 122 changes in size and adjusts an excess fuel flow rate of fuel returned to the fuel tank through the return pipe 132 and as a result the fuel pressure in the fuel chamber 112a is kept constant.
However, in the fuel supply system using the pressure regulator shown in FIG. 13, when after the engine is stopped the engine is restarted again while it is still hot, vapor arises in the fuel in the supply passage from the fuel pump to the injectors. When this fuel including vapor is supplied to the injectors the fuel injection quantity decreases and consequently the engine may not start well or may not start at all.
To solve this kind of problem, in a fuel supply system disclosed in the international (PCT) patent application, Publication No. H5-500099, the fuel delivery flow rate from the fuel pump is made adjustable according to the fuel flow rate demand from the engine. When the return flow rate of fuel from the pressure regulator is between Q.sub.min and Q.sub.max, a control pressure, that is, the pressure at which fuel is supplied to the injectors, is kept constant as shown in FIG. 9. The control pressure can be increased by increasing the fuel delivery flow rate from the fuel pump so that the return flow rate exceeds Q.sub.max. In this way, because during hot restarting of the engine it is possible to raise the control pressure by increasing the fuel delivery flow rate from the fuel pump, the production of vapor caused by the high temperature can be suppressed by increasing the pressure of the supplied fuel.
However, in this conventional fuel supply system, in the control pressure range over which the occurrence of vapor can be suppressed, the control pressure varies greatly depending on the return flow rate of fuel, i.e., on the fuel delivery flow rate from the fuel pump. Consequently, control of the control pressure is difficult and there has been the problem that, when the fuel supply flow rate to the injectors varies, the air/fuel ratio fluctuates. Also, there is a likelihood of fuel leakage from the fuel piping occurring due to the control pressure increasing too much.
Besides this, there have been [1] apparatuses which suppress the occurrence of vapor by raising the valve-opening pressure of the pressure regulator when the engine is stopped or when the fuel pump is stopped; [2] apparatuses which suppress the occurrence of vapor by providing a passage cutoff valve in an outlet of a fuel supply pipe and closing this passage cutoff valve and thereby closing the fuel supply passage when the engine is stopped; [3] apparatuses which suppress the occurrence of vapor by providing two pressure regulators, one for low pressure and one for high pressure; and [4] apparatuses which operate two fuel pumps and forcibly discharge vapor into the return pipe.
In the fuel supply systems described in [1] to [4] above, with those in which excess fuel is returned to the fuel tank by a return pipe, because the construction is complicated and hot fuel is returned to the fuel tank, there has been the problem that the temperature of the fuel in the fuel tank rises and evaporation tends to occur.
As proposed in Nakashima U.S. Pat. No. 5,359,976, a fuel supply system is being considered wherein to alleviate this the excess fuel return pipe is dispensed with, all connectors through which fuel is distributed from the fuel supply pipe to the injectors of engine cylinders are extended to an upper part of the fuel supply pipe, the fuel inlet openings of the injectors open into this upper part of the fuel supply pipe, a branched fuel pipe is disposed from the upstream side of the fuel supply pipe to the upper part of the fuel supply pipe, the fuel pump is connected to the fuel supply pipe by a connecting constriction and as a result vapor having arisen does not directly affect the fuel injection quantity during engine starting. However, this apparatus will also has a likelihood that the fuel supply piping is complex and large.